dt-bindings: iommu: arm,smmu: document the SM8650 System MMU

Document the System MMU on the SM8650 Platform.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20231128-topic-sm8650-upstream-bindings-smmu-v2-1-d3fbcaf1ea92@linaro.org


Signed-off-by: Will Deacon <will@kernel.org>